Levers

A lever is a stiff bar that turns about a fixed point called the fulcrum. Lay a ruler over a pencil: a light eraser on one end can lift a heavy stapler on the other.

The parts of a lever:

• Fulcrum: the point it turns about.
• Load: the force you want to overcome.
• Effort: the force you apply.
• Load arm and effort arm: the distances of the load and the effort from the fulcrum.

The balance rule. The end with the small force moves a long way; the end with the big load moves a little. The lever balances when:

effort × effort arm = load × load arm (F₁d₁ = F₂d₂).

So MA = effort arm ÷ load arm: the longer your arm, the less effort you need. You can check this with coins on a home-made beam balance.

A seesaw. Two children of the same weight balance when they sit at the same distance from the middle. If one is heavier, the heavier child must sit closer to the middle. A 15 kg child 2 m from the fulcrum balances a 30 kg child 1 m away: 15 × 2 = 30 × 1.

Try a door. Push it near the hinges (the fulcrum): it is hard. Push at the far edge: easy. That is why the handle is on the far edge.

Three classes of lever:

Two everyday uses: a spoon lever opens a tight tin lid; and to cut something hard with scissors, put it close to the fulcrum (the pivot).

Can a machine run forever on its own? No. Machines don't create energy, and real machines always lose some to friction. So a perpetual motion machine is impossible.